Tilted slot antenna



July3i, 1951 I H, J, RIBLET 2,562,332

TILTED SLOT ANTENNA Filed May 5, 1945 FIG.1A

EY Q PTR6AE/Ifi6r OF ENERGY OUT O PAPER FIG. 2.

TRANSMITTER RECEIVER INVENTOR. HENRY J. RIBLET BY I WWW QM ATTORNEY Patented July 31, 1951 TILTED SLOT ANTENNA Henry J. Riblet, Cambridge, Mass., assignor, by

mesne assignments, to the United States of v America as represented by the Secretary of War ApplicationMay 3,1945, Serial No. 591,744

7 Claims.

The present invention relates to ultra-high frequency antennas and more particularly to antennas having circular polarization.

There exist numerous applications entailing the use of an ultra-high frequency, omni-directional antenna. For example, in radio beacons, it is desirable that the radiation pattern be circular so that the field strength for a given distance from the antenna is substantially the same for any direction with respect to the antenna.

Also an antenna having the characteristics for producing circularly polarized-radiation may, for example, be used as a beacon capable of operating in connection with either vertically or horizontally polarized communication systems including radio object-locating systems.

It is the primary object of this invention to provide an antenna in the form of a slotted wave guide or coaxial line which is adapted to radiate circularly polarized electromagnetic waves of energy.

Another object of the invention is to provide a slotted wave guide or coaxial line antenna system'which is substantially omni-directional in one plane and which is relatively sharply directional (to a controllable extent, if desired) in a plane-perpendicular to the first-mentioned plane, and moreover in which the energy waves radiated therefrom are circularly, vertically or horizontally polarized depending on the angular relation of the slots relative to the longitudinal axis of the wave guide or coaxial line.

It will be understood that polarization as referred to in this description and the appended claims is defined in the sense used in the radio art rather than in the optical sense, and refers to the direction of the electric vector rather than that of the magnetic vector.

Generally stated, these objects are attained by the use of an antenna structure in the form of a coaxial line or wave guide ringed by a plurality of similarly tilted slots. A pair of parallel reflecting plates are externally arranged with respect to the slots whereby a wave guide effect is realized. The orientation of the slots are such that a 90-degree phase shift is obtained between the voltage vector components of energy radiated from the slots, thereby producing a substantially circular radiation pattern.

For a better understanding of this invention, as well as other objects and features thereof, reference is had to the following detailed description to be read in connection with the accompanying drawing wherein: I

Fig. 1 shows in perspective a preferred embodiment of an antenna structure in accordance with the invention,

Fig. 1A shows a sectional view of Fig. 1 together with an illustration of the source and form of the wave supplied thereto, and

Fig. 2 is a voltage vector diagram of energy propagated from one slot in the antenna structure of Fig. 1.

Referring now to the drawing and more par ticularly to Fig. 1, there is illustrated, by way of example, a circular polarization antenna comprising a wave guide It), having a circular cross section, connected to the output of a transmitter or receiver apparatus as shown in Fig. 1A. Wave guide it has a plurality of tilted slots ll cut therein at equi-spaced positions along the circumference thereof, and a pair of annular, parallel, reflecting plates [2 and I3 encircling guide I0 above andbelow the area of slots II. The reflecting plates l2 and [3 are spaced at least a half wavelength and preferably not more than a wavelength apart. Plates I2 and [3 are sufiiciently large and extend radially for a distance preferably more than a quarter wavelength from the wave guide In to produce an appreciable wave guide effect on the energy radiated from slots II which are preferably tilted at approximately 60 degrees from the horizontal when wave guide H] is in its contemplated vertical position. If desired a series of such slots and plates may be spaced along the length of wave guide H1 in proper phase relationship to produce a combined pancake type beam pattern.

The theory underlying the invention may be understood by referring to Fig. 2 showing a vector diagram of the energy radiated from a slot II.

The direction of propagation from slot 1 l is perpendicular to the plane of the drawing. Vector E, therefore, represents the voltage of radiated energy, being perpendicular to the slot.

Considering operation of the antenna from the transmitting point of view it will be evident that the radiation energy issuing from slot H must have plane polarization in a direction perpendicular to the axis of the slot as represented by vector E; otherwise the slot will not radiate energy. If the energy in the guide is of a mode which can be resolved to have an electric component in that direction, it will be suitable for feeding the guide. Furthermore, the electric component should be symmetrical around the inner circumference of the guide to radiate effectively from all the slots which symmetrically encircle the guide.'

Conversely from the receiving point of view the slot can only accept energy having a component of its electric vector in a direction perpendicular to the slot as shown by the vector E.

It follows that within the circular guide shown, the preferred mode that can be employed is the TM0,1 mode. Its rectangular guide counterpart, the TM1 ,1 mode, could be used with a slotted rectangular guide.

The 'IE1,1 fundamental mode in the circular guide would be ineffective since it is unsymmetrical and would provide radiation from only a part of the slots which symmetrically encircle the guide shown in Fig. 1 and hence would not be employed. However, its rectangular guide counterpart, TEo,1 mode can and would preferably be employed in a rectangular wave guide where the slots ll would be located in only the wider surfaces of the guide.

The terminology for modes in wave guides as here employed is that given in Radar Electronic Fundamentals, published by the Bureau of Ships of the Navy Department, page 369.

Fig. 1A is a cross section view of Fig. 1 and illustrates the introduction of a wave of the preferred fundamental mode TMo,1 supplied by generator 14 via connections to the wall of the guide and to an exciting element I5. The conventional representation for magnetic and electric lines of force within the guide is employed in this illustration.

For purposes of illustration, vector E is divided into two component vectors-Ez and Ey. Since vector Ez lies in the plane of parallel reflectors l2 and I 3, it is effectively contained in a wave guide and will no longer have free space velocity. Consequently, vector Ez will lag vector Ey. There will also occur a slight amount of attenuation of vector Ez due to conduction losses, with the net result, if proper adjustments are made in the angle of slot II, that vector E3; and Ez will be 90 degrees out of phase with each other and of equal absolute magnitude. This results in a circular polarized wave radiating from the antenna with a complete 360 degree azimuth coverage. It will be seen that different types of horizontal or ver- ,tical polarization elfects may be obtained by varying the tilt of slots H. In other words various :degrees of elliptical polarization may be had; the most desirable degree, of course, being the circular polarization as described above.

Although the invention has been illustrated in a preferred embodiment entailing a circular wave guide [0, it is to be understood that the invention is operable with a rectangular guide or a coaxial line and external reflecting plates.

While there has been described what is at present considered a preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In an antenna arrangement, the combination comprising a wave guide, having a slot therein, connected to a source of wave energy, and a pair of parallel reflector plates arranged with said slot therebetween so as to have a wave guide effect on energy radiated therefrom, said slot being tilted at an angle relative to the planes of said plates different from 90 and whereby one of the vector components of the voltage vector of energy propagated from the slot lies in the plane of said parallel plates.

2. A circularly polarized antenna comprising a wave guide having a plurality of tilted slots equi-spaced around the periphery thereof, and a pair of reflector plates arranged in parallel above and below said slots at least one-half wavelength apart at the operating frequency, said slots being tilted at an angle different from and 0 relative to the planes of said plates whereby one of the vector components of the voltage vector of energy propagated from the slot lies in the plane of said parallel plates.

3. A circularly polarized antenna comprising a circular wave guide having a plurality of tilted slots equi-spaced around the circumference thereof, and a pair of annular reflecting plates encircling said wave guide arranged in parallel above and below said slots at least one-half wavelength apart at the operating frequency, said slots being tilted at an angle different from 90 and 0 relative to the planes of said plates whereby one of the vector components of the voltage vector of energy propagated from the slots lies in the plane of said parallel plates.

4. A circularly polarized antenna comprising a waveguide having a plurality of tilted slots equi-spaced around the circumference of the waveguide, means for supplying microwave energy to said guide, said energy having an electric vector component normal to the axes of said slots, and a pair of annular reflecting plates encircling said waveguide arranged in'parallel above and below said slots at least one-half wavelength apart at the frequency of said microwave en.- ergy, said slots being tilted at an angle different from 90 and 0 relative to the planes of said plates whereby one of the vector components of the voltage vector of energy propagated from the slots lies in the plane of said parallel plates.

5. An antenna comprising a circular wave guide having a plurality of slots equi-spaced around the circumference thereof, and a pair of annular reflecting plates secured around said wave guide arranged parallel to each other above and below said slots at least one-half wavelength apart at the operating frequency, the radial width of said plates being more than a quarter of the wavelength of said operating frequency, said slots being tilted at an angle of substantially 60 degrees relative to the cross-sectional plane of said wave guide whereby one of the vector components of the voltage vector of energy radiated from said slots lies in the plane of said parallel plates, said vector component being so afiected by said plates and attenuated that the same will be 90 degrees out of phase with and of equal magnitude to the other voltage vector component whereby circularly polarized radiation will be produced.

6. An antenna structure for transmitting and receiving elliptically polarized waves which may be defined from the transmission point of view, as comprising a waveguide portion having a slot shaped aperture for radiating a plane polarized wave, means comprising a pair of waveguide surfaces spaced apart by at least a half of the operating wave length and disposed about said slot at an angle different from 90 and 0 relative to the direction of said slot for changing the velocity of a component of said wave over a predetermined length of path, said surfaces extending from said waveguide sucha distance as to provide a time quadrature displacement of said component thereby effectively to radiate an elliptically polarized wave.

7. An antenna fortransmitting and receiving elliptically polarized electromagnetic wave energy, comprising a waveguide provided with a slot and a pair of parallel plates positioned adjacent said slot. and with said slot therebetween and with the planes of said plates at an angle to the length of said slot difierent from 90 and 0 whereby said plates have a waveguide effect on a vector component only of the electromagnetic energy radiated from said slot and said vector component is retarded with respect to the remainder of the radiated electromagnetic energy for producing elliptical polarization.

HENRY J. RIBLET.

REFERENCES CITED The following references are of record in the file of this patent:

Number Number 6 UNITED STATES PATENTS Name v Date Green Sept. 15, 1903 Southworth July 9, 1940 Dallenbach May 6, 1941 Dallenbach May 30, 1944 Carter Dec. 10, 1946 Lindenblad Jan. 14, 1947 Southworth Feb. 8, 1949 FOREIGN PATENTS Country Date Great Britain Jan. 16, 1935 France May 8, 1939 

